amps or 1 amp below the normal threshold Since continu-

ous operation at 3 amps may overheat the injectors the

timer function on the IC will force the transition into the hold

state after one time constant (the time constant is equal to

systems where the timer function is not needed it can be

disabled by grounding Pin 8 For systems where the initial

peak state is not required (ie where the solenoid current

rises immediately to the hold level) the timer can be used to

disable the peak function This is done by setting the time

recommended The timer will then complete its time-out and

disable the peak condition before the solenoid current has

had a chance to rise above the hold level

The actual range of the timer in injection systems will proba-

bly never vary much from the 39 milliseconds shown in

Figure 1 However the actual useful range of the timer ex-

tends from microseconds to seconds depending on the

stray capacitances for low values and by leakages for large

values

The capacitor reset time at the end of each controller pulse

is determined by the supply voltage and the capacitor value

discharging it with a current of approximately 15 mA Thus

a 01 mF cap is reset in approximately 25 ms

COMPENSATION

Compensation of the error amplifier provides stability for the

circuit during the hold state External compensation (from

Pin 2 to Pin 3) allows each design to be tailored for the

characteristics of the system andor type of Darlington pow-

er device used In the vast majority of designs the value or

type of the compensation capacitor is not critical Values of

100 pF to 01 mF work well with the circuit of

Figure 1 The

value shown of 01 mF (disc) provides a close optimum in

choice between economy speed and noise immunity In

some systems increased phase and gain margin may be

appropriately rated 01 mF capacitor This is however rarely

necessary

FLYBACK ZENER

time the injector current is reduced This occurs at the peak-

to-hold transition (when the current is reduced to one fourth

of its peak value) and also at the end of each input pulse

(when the current is reduced to zero) The zener provides a

current path for the inductive kickback limiting the voltage

voltage levels Thus the rated zener voltage at the system

peak current must be less than the guaranteed minimum

majority of the injector current during the peak-to-hold tran-

sition (see

level This fact is easily overlooked and as described in the

following text can be corrected if necessary Since the error

biased to provide exactly that Thus the safe operating area

volts For systems where this is not desired the zener an-

ure 5 Since the voltage across the sense resistor now ac-

curately portrays the injector current at all times the error

TLH5062 – 7

FIGURE 4 Circuit Waveforms

to the proper value The disadvantage of this particular con-

figuration is that the ungrounded zener is more difficult to

heat sink if that becomes necessary

protection Automotive systems are susceptible to a vast

array of voltage transients on the battery line Though their

This is one reason why a zener is preferred over a clamp

diode back to the battery line the other reason being long

decay times

TLH5062 – 8

6